External cavity laser source

ABSTRACT

A tunable laser source that includes multiple gain elements and uses a spatial light modulator in an external cavity to produce spectrally tunable output is claimed. Several designs of the external cavity are described, targeting different performance characteristics and different manufacturing costs for the device. Compared to existing devices, the tunable laser source produces high output power, wide tuning range, fast tuning rate, and high spectral resolution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority of applicationSer. No. 13/154,516, filed on Jun. 7, 2011, which itself claims priorityof Provisional Patent Application Ser. No. 61/358,135, filed on Jun. 24,2010. The contents of the priority applications are incorporated hereinby reference.

GOVERNMENT RIGHTS

This invention was made partially under Phase I SBIR Contract No.:D11PC20141 awarded by the Department of Homeland Security. Thegovernment has certain rights in the invention.

FIELD

This disclosure is in the technical field of tunable lasers, moreparticularly, tunable lasers containing an external cavity. Furthermore,it is in the technical field of laser spectroscopy with advancedexcitation waveforms.

BACKGROUND

A common way to force a tunable laser gain medium to produce output at aspecific wavelength is to couple the gain medium to an external lasercavity. The external cavity disperses the intracavity radiation into itsspectral components and selects the wavelength whose oscillation in thelaser cavity is sustained. Typically, this wavelength has the highestgain/loss ratio among the competing wavelengths (cavity modes).

External cavity tuning is generally the preferred way of laser tuningwhen wide tuning range is required. Besides wide range, it provideswavelength reproducibility and stability over time. High tuning speed,however, has been difficult to achieve simultaneously with a wide tuningrange, with external cavity designs in the past.

FIG. 1a illustrates the traditional Litrow design of an external cavity10, with gain element 12, lens 14 and diffraction grating 16. In thiscase, the laser power is out coupled from the cavity through the zero-thorder reflection of grating 16 [e.g., Maulini, 2006]. Laser tuning isachieved by grating rotation. In this simplest form of the Litrowdesign, tuning by grating rotation has an additional disadvantage thatit changes the direction of the output beam. Laser tuning by mechanicalrotation of the grating, although amenable to computer control [e.g.,Ignjatijevic and Vujkovic-Cvijin et al., 1985], is clearly notwell-suited for fast laser tuning.

An approach which makes external cavity tuning both fast and digitallyprogrammable, while retaining the advantages of external cavity tuning,makes use of a spatial light modulator (SLM) located inside the cavity,to spatially modulate the reflectivity of one of the “cavity mirrors.”With a dispersive device inside the cavity, the spatial modulation ofthe SLM translates into spectral modulation, resulting in lasing actionat the desired wavelength. A device which uses a SLM for external cavitytuning has been described previously by Gutin [Gutin, 2001, Gutin, 2003]and by Breede et al. [Breede et al., 2007].

In a typical example of such a cavity 20 shown on FIG. 1b , the lasergain element 22 is anti-reflection (AR) coated at the facet facing theexternal cavity, while the coating of the opposite facet is suitable forlaser beam out coupling. The light emerging from the AR coated facet iscollimated with a high numerical aperture (NA) lens 24 and directed to adiffraction grating or other dispersive element 26. The first-orderdiffracted beam is captured by a lens (not shown) and directed towardsthe SLM 28, typically represented by a Digital Micromirror Array (DMA)[Hornbeck, 1991, Digital Light Innovations, 2011]. Dispersed beams ofdifferent wavelengths are spatially resolved on the surface of the DMAwhere individually addressable micromirrors select the wavelength whichis sent back to oscillate in the cavity. The DMA operates by tilting itsmicromirrors between two predetermined stable states, under digitalelectronic control. In one of the states, the micromirrors reflect thebeam back to the gain element (the “on” state), while in the otherstate, the light is decoupled from the cavity (the “off” state). Thewavelength corresponding to the spatial position on the DMA withmicromirrors turned “on” will oscillate in the cavity. As a consequence,laser tuning under fast digital electronic control inherent to the DMAmodulator becomes possible. Since the digitally controlled externalcavity laser has the ability to turn on or off any wavelength in anyorder, any laser wavelength can be accessed at random, as opposed tosequential wavelength tuning. Random access tuning is a unique featureof digitally controlled external cavity laser tuning.

SUMMARY

This disclosure describes an approach which in one non-limitingembodiment uses multiple gain elements that operate within a singleexternal cavity, producing a laser source with a wider tuning range andhigher output power than heretofore possible. The disclosure alsodescribes an external cavity fabricated, completely or partially, out oftransparent solid state optical material which makes the laser sourceresistant to external disturbances and therefore more practicallyuseful.

This disclosure comprises in one embodiment the use of multiple gainelements within the same laser cavity. This disclosure also describes aparticularly advantageous approach to realizing external laser cavities,based on building a cavity, completely or partially, inside a block oftransparent optical material. Well-corrected wide-field imagingspectrometers with SLM modulators can be used as the foundation forexternal laser cavity designs that allow placement of multiple gainelements in a single cavity. Due to sufficiently wide field, theseelements experience essentially the same feedback from the cavity, whichmakes any gain element capable of producing lasing action at the desiredwavelength selected by the SLM.

This disclosure also describes the use of fast non-sequentially tunable(random wavelength access) laser to construct spectral output tailoredto enhance the sensitivity of spectroscopic detection of materials andfeatures partially obscured by the background or by interfering species.

Advantages of the Present Invention Include, without Limitation, One orMore of the Following:

(1) external laser cavity with simple and robust optics;

-   -   (2) external laser cavity operating simultaneously with multiple        laser gain elements, each independently tunable;    -   (3) wider tuning range than existing designs, due to multiple        gain elements;    -   (4) higher output power per wavelength, when multiple lasers are        tuned to the same wavelength;    -   (5) higher total laser output power;    -   (6) fast random access to any wavelength within the gain medium        spectrum;    -   (7) enhanced spectroscopic detection sensitivity due to        in-hardware processing during data acquisition;    -   (8) enhanced spectroscopic detection sensitivity due to advanced        wavelength modulation techniques used to generate the signal        corresponding to the spectral features of the analyte and not to        those of the background or interferents.

This disclosure features a tunable laser containing an external cavitycomprising multiple tunable gain elements providing sources of opticalradiation at different wavelengths, a spectral dispersion deviceproducing radiation dispersed into separate wavelengths, a spatial lightmodulator (SLM) defining a plurality of sub-apertures that are adaptedto change their reflectivity, transmissivity or diffraction properties,one or more optical elements that provide intracavity beam shaping andimaging, and a control system that causes the SLM sub-apertures tochange their properties, to determine the output wavelength of thelaser.

The multiple tunable optical gain elements may comprise devicesstructured as semiconductor diodes, quantum cascade devices orinter-sub-band devices. The multiple tunable optical gain elements maycomprise crystals, dyes, or may be in the gaseous state. The dispersionelement may be diffractive, refractive, or a combination of refractiveand diffractive elements. The dispersion element may be diffractiveeither in reflection or transmission. The dispersion element maycomprise one or more spectral filters, and may comprise an array ofspectral filters. The SLM may comprise a digital micromirror array.

The spatial light modulator may comprise a micromirror array, amicromechanical ribbon array, a liquid crystal array, or an array ofapertures that are adapted to open and close under digital control. Thespatial light modulator may comprise microshutters. The tunable lasermay further comprise a solid optically transparent object that definesat least part of the volume of the cavity. The optically transparentobject may comprise multiple blocks that are coupled together to createthe cavity. The spectral dispersion device and the SLM may each bemounted to a block.

The control system may be enabled to automatically perform spectroscopicsignal processing on the SLM by applying a mathematical transform togenerate a set of output laser wavelengths in order to enhance thedetection sensitivity of the system, or reduce the time needed toperform chemical analysis or material identification, or reduce laserpower requirements needed to perform chemical analysis or materialidentification. The control system may control the SLM to accomplishtime-multiplexed modulation of multiple laser wavelengths or a matchedfilter or derivative spectroscopy or derivative spectroscopy withnon-adjacent wavelengths. The control system may be enabled toautomatically perform spectroscopic signal processing on the SLM byapplying a mathematical transform to generate a set of output laserwavelengths in order to enhance the detection sensitivity of the system,or reduce the time needed to perform chemical analysis or materialidentification, or reduce laser power requirements needed to performchemical analysis or material identification, and the control system maycontrol the SLM to accomplish time-multiplexed modulation of multiplelaser wavelengths or a matched filter or derivative spectroscopy orderivative spectroscopy with non-adjacent wavelengths.

This disclosure also features a tunable laser containing an externalcavity comprising one or more tunable gain elements providing sources ofoptical radiation at different wavelengths, a spectral dispersion deviceproducing radiation dispersed into separate wavelengths, a digitalmicromirror array (DMA) that defines a plurality of sub-apertures thatare adapted to change their reflectivity, transmissivity or diffractionproperties, one or more optical elements that provide intracavity beamshaping and imaging, a solid optically transparent object that definesat least part of the volume of the cavity, and a control system thatcauses the sub-apertures of the DMA to change their reflectanceproperties, to determine one or more output wavelengths of the laser.

The control system may be enabled to automatically perform spectroscopicsignal processing on the DMA by applying a mathematical transform togenerate a set of output laser wavelengths in order to enhance thedetection sensitivity of the system, or reduce the time needed toperform chemical analysis or material identification, or reduce laserpower requirements needed to perform chemical analysis or materialidentification. The control system may control the DMA to accomplishtime-multiplexed modulation of multiple laser wavelengths or a matchedfilter or derivative spectroscopy or derivative spectroscopy withnon-adjacent wavelengths. The optically transparent object may comprisemultiple blocks that are coupled together to create the cavity. Thespectral dispersion device and the SLM may each be carried by at leastone block.

This disclosure further features a tunable laser containing an externalcavity comprising one or more tunable gain elements providing sources ofoptical radiation at different wavelengths, a spectral dispersion deviceproducing radiation dispersed into separate wavelengths, a spatial lightmodulator (SLM) defining a plurality of sub-apertures that are adaptedto change their reflectivity, transmissivity or diffraction properties,one or more optical elements that provide intracavity beam shaping andimaging, and a control system that causes the SLM sub-apertures tochange their properties, to determine the output wavelength of thelaser, wherein the control system causes the creation of a series ofspecific wavelength bands selected to detect or quantify a specificanalyte based on laser light absorption, transmission or reflectance soas to discriminate the spectral features of the analyte from those ofthe background or interfering species.

The control system may implement a matched filter for the analyte viaselection of appropriate wavelengths and weighting factors. The controlsystem may cause the SLM sub-apertures to change their properties so asto modulate at least two or more of the specific wavelengths atdifferent frequencies.

Further featured are an infrared spectrometer comprising the tunablelaser described herein, an infrared spectral imaging microscopecomprising the tunable laser described herein, a standoff spectroscopicdetection instrument comprising the tunable laser described herein, anda trace gas spectrometer comprising the tunable laser described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects features and advantages will occur to those skilled in theart from the following description of embodiments and the accompanyingdrawings, in which:

FIG. 1a is a schematic diagram of a prior art tunable external cavitylaser in which tuning is accomplished with grating rotation.

FIG. 1b is a schematic diagram of a prior art tunable external cavitylaser in which tuning is accomplished with a spatial light modulator(SLM) located in the external cavity.

FIG. 2 is a schematic diagram of a tunable external cavity with adigitally controlled intracavity SLM and with multiple gain elements.

FIG. 3 is a schematic diagram of an external laser cavity with a singleaspheric concave grating and wavelength tuning with a SLM.

FIG. 4a is a schematic diagram of a dispersive Offner relay used forexternal cavity laser tuning with a SLM.

FIG. 4 b is a schematic diagram of a dispersive Dyson relay used forexternal cavity laser tuning with a SLM.

FIG. 5 is a schematic diagram of an external laser cavity with SLMtuning based on the crossed Czerny-Turner spectrometer design.

FIG. 6 is a schematic diagram of an external laser cavity with SLMtuning and with a lensed prism as the dispersing element.

FIG. 7 is a schematic diagram of a solid-state external laser cavitybased on the Offner dispersive relay design with SLM wavelength tuning.

FIGS. 8a and 8b show a hypothetical example of a spectral signatureconsisting of an analyte spectrum superimposed on a background spectrum.

FIGS. 8c and 8d show a hypothetical example of a matched filter,including one in a discrete form suitable for use with tunable lasers.

FIG. 9 is a schematic of a generalized laser spectrometer.

FIG. 10 is a schematic of a generalized laser illuminated microscope.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 2 shows a general representation corresponding to several externalcavity designs 30 with computer-controlled digital tuning (using userinterface 46 and controller 44) made possible by a reflectiveintracavity SLM 32. The gain medium 40 can be a semiconductor device(e.g. a diode laser, a quantum cascade laser, an inter-sub-band laser),a crystal (typically optically pumped), a dye (typically opticallypumped), a gas, or any other tunable laser medium. The dispersiveelement 34 in the cavity can be a diffraction grating, a prism, a grism(combination prism and grating), one or more spectral filters, or acombination thereof. Lenses 36 and 38 focus and image the light in thecavity. Lens 42 collimates the laser output. The SLM inside the cavitycan be a micromirror array (DMA), a reflective or diffractiveribbon-like array [Trisnadi, et al., 2004, Senturia et al., 2005], aliquid-crystal array, an array of programmable apertures (e.g.,microshutters [Li et al, 2010]) or any other one-dimensional ortwo-dimensional SLM. The choice of elements used depends on the intendedapplication, some of which are described herein. Each choice ofintracavity elements has its own advantages and disadvantages relativeto the performance, size, weight, and cost of the final device.

FIG. 2 shows multiple gain elements 40 stacked vertically in the planeperpendicular to the plane of the drawing. In a well-designed cavity,optical losses are similar for a large number of elements. In theparticular case of semiconductor gain media, up to several tens ofelements can be stacked close to each other, especially in the case whenthese are fabricated as waveguides on a single semiconductor wafer, asis customary in semiconductor manufacturing. The array of lasersfabricated on a single substrate is mounted “vertically”, with laserslocated one above the other, in the external cavity shown on FIG. 2. Dueto their origin on multiple facets, output beams from individual gainelements will be displaced spatially in the “vertical” direction, in theamount corresponding to the distance between the facets. For mostpractical applications this displacement will be small enough to beeither negligible or easily correctable. In non-limiting examples, thecorrecting optical system can be based on a fiber combiner or adiffraction grating with suitable optics, as is well known in the art.

With multiple lasers operating within a single cavity and independentlytuned with a two-dimensional DMA, a wavelength-multiplexed laser sourcecan be constructed. Such source provides both wide tuning range and highpower output, both at individual wavelengths and in total output. In thecase of a DMA as the SLM in the cavity, the modulation bandwidth reachesup to 32 kHz with commercially available DMAs [Digital LightInnovations, 2011]]. This bandwidth is high enough for most laserspectroscopy applications, since it is above the bulk of thelow-frequency environmental noise.

Alternative Embodiment 1

FIG. 3 shows conceptually the simplest reflective design of cavity 50with gain elements 52. A single optical element 54 performs bothdispersion and imaging. In this design, a grating is inscribed on ahighly corrected concave aspheric surface which both disperses andimages the laser facets on the SLM 56. This approach poses amanufacturing challenge related to the aspheric element (close to aparaboloid). For a subset of these designs, multi-axis single pointdiamond turning (SPDT) manufacturing technology is capable of deliveringthe required performance, particularly at relatively long IR wavelengths(LWIR portion of the spectrum).

Alternative Embodiment 2

Designs of cavities based on concentric imaging spectrometers areillustrated in FIGS. 4a and 4b . A cavity 60 based on the Offnerdispersive relay is shown on FIG. 4a while a cavity 130 based on Dysonrelay is shown on FIG. 4b . The Offner configuration 60 uses a concavemirror 64 and a convex grating 68 to disperse and reimage the facet ofthe gain element 62 on the SLM 66. The Dyson configuration 130 uses arefractive block 134 and a concave grating 136 to disperse and reimagethe facet of the gain element 132 on the SLM 138. The concentric designis capable of providing high numerical aperture to collect the laseroutput. It can work well with spherical-only surfaces, which makes iteasier to manufacture.

Alternative Embodiment 3

A design of cavity 70 based on the crossed Czerny-Turner spectrometer isshown on FIG. 5. This design has the advantage of using a plane grating78, which relaxes the manufacturing complexity and cost. Two concaveaspheric surfaces of high numerical aperture 74 and 76 collect and,together with a plane grating, disperse and reimage the laser facets 72on the face of the SLM 80. The manufacturing challenge here is reducedrelative to the Alternative Embodiment 1 because the manufacturing ofaspheric surfaces is decoupled from the manufacturing of diffractivesurfaces.

In cases with reflective gratings the initial prototyping cost can behigh, but reflective machined surfaces can be subsequently preciselyreplicated at low cost for volume production.

Alternative Embodiment 4

An external cavity design 90 based on refractive instead of reflectiveoptical elements is shown on FIG. 6. The cavity contains anantireflection (AR)-coated lensed prism 96 (a prism with curvedsurfaces) to both disperse and image the laser facet on gain element 92.Lens 94 is used to focus and image the light. The system images theoutput facet of the gain element on the surface of the SLM 100 with lowdistortion and with high spectral dispersion. (The inherently nonlinearwavelength-to-angle dependence of the prism dispersion can be easilycorrected through a calibration routine in the spectrometer controlsoftware). The system contains only three optical elements besides thelaser and the SLM, providing both simplicity and low internal cavitylosses. The surfaces of the curved prism 96 are all spherical.

In a preliminary design, an aberration-corrected lens collimates thelaser output beam and sends it through the lensed dispersion prism. Ahigh-reflection (HR)-coated mirror reflects the beam back through theprism. The lensed prism focuses the dispersed beam onto the surface ofthe SLM modulator which sends the selected wavelength back to the laserfacet. The overall size of the complete external laser cavity isfavorable for practical external-cavity lasers. All optical surfaces inthis system are either AR-coated (lens and prism) or HR-coated (mirror),with high efficiency broadband coatings.

Alternative Embodiment 5

External Laser Cavity Fabricated Out of Transparent Solid State OpticalMaterial

The external laser cavity can be built entirely or partially out ofsolid state material transparent in the spectral range of laseroperation. In this design, the laser gain element and the SLM aredirectly attached to the cavity block. In one embodiment, the opticalschematic of the cavity follows the principles of the Offner dispersiverelay (FIG. 4a ). More specifically, the cavity design 110 shown on FIG.7 relies on the use of two solid pieces of transparent material (block112 and block 114) into which the optical elements of the Offnerconcentric dispersive relay are machined by using e.g., the single pointdiamond turning (SPDT) manufacturing technique. The cavity is machinedas two adjoined blocks joined at location 116. The cavity contains ahigh-reflection coated mirror (the rear face 113 of block 112), machineddiffraction grating 118, and anti-reflection coated interfaces 120, 122to which the laser gain element and the SLM are attached, respectively.

A solid state laser cavity can be also developed to accommodate multiplegain elements within a single external cavity. Each gain element can beindependently tuned with a two-dimensional DMA, resulting in optimizedtuning range/output power combination from a single solid state cavity.This design represents a foundation for a compact tunable laser sourcewith exceptional shock and vibration resistance due to its monolithicsolid state construction with no macroscopic moving parts. Additionally,the design removes the need for having an optical window, typicallyfound on a SLM, thereby reducing intracavity losses and the cost of thelaser.

Alternative Embodiment 6

Digitally Tunable External Laser Cavity of Alternative Embodiments 1-3Coupled with on-Chip (in-Hardware) Processing for Data Acquisition

The fast digitally tunable and modulatable laser source represents afoundation for a laser spectrometer that extends the idea of classicalwavelength modulation absorption spectroscopy towards a “multiplexed”laser spectroscopic technique capable of improved detection sensitivity.The approach is to shorten the data acquisition cycle by reducing thenumber of wavelengths required for detection. Two possible approaches todeveloping the optimum combination of wavelengths are presented here.

The first approach is to reduce the number of wavelengths to bemeasured, confining them to spectral regions of interest for thespecific analyte and for expected interferents and background features.These regions need not be contiguous. Once the wavelengths areidentified, the absorbances can be measured by the digitally tunablelaser spectrometer in a very short time.

A second, possibly more powerful, approach, is to devise a sequence ofwavelengths that, together with appropriate weighting factors,implements a “matched filter” for the analyte in the given spectralrange. The matched filter is a filter vector that extracts, via linearprojection, the analyte signal with optimal rejection of additiveGaussian clutter background, as characterized by a covariance matrix. Itis a well-known technique for identifying a known spectral componentbelonging to a target analyte within a medium containing interferingspectral background (a hypothetical example shown on FIGS. 8a and 8b ).The matched filter is a linear filter vector, i.e. a spectrum, whose dotproduct with the measured spectrum produces a value representing thestrength of the analyte features present in the measured spectrum. Thematched filter for an analyte contained within a background medium isthe spectrum that gives the smallest rms value when applied to a scenethat does not contain the analyte. See FIG. 8c . Thus the matched filteris the best linear filter, in the least square sense, for detecting atarget analyte within the background. A matched filter can beconstructed from two spectral templates, one representing the portion ofthe matched filter spectrum with positive values, and one representingthe portion of the matched filter with negative values. See FIG. 8d .The application of the matched filter method with a tunable externalcavity laser relies on generating a set of laser lines at prescribedwavelengths with prescribed intensity that produce the compositespectroscopic signal measured by the instrument. The matched filter“score” may be determined by making measurements in rapid successionwith each of the spectral templates, and subtracting the twomeasurements. The discrete wavelength-intensity pairs on FIG. 8bconstitute the “prescription” for the detection of the analyte withinthe analyte/background combination shown on FIG. 8a . With digitallycontrolled external cavity lasers sets of sequential laser wavelengthscan be generated at high speed corresponding to the SLM frame rate (>30kHz for a DMA).

The matched filter approach is advantageous when it can be effectivelyimplemented with fewer wavelengths than the method that takes anend-to-end spectrum. These detection methods perform a part of thedetection process in the data collection step, within the programmablehardware portion of the sensor. Therefore it has the advantages typicalof “compressed sensing”: the balance between the sensitivity, falsealarm rate and the time to take a measurement is improved by notburdening the instrument with collecting and analyzing redundant and/orirrelevant data.

Alternative Embodiment 7

Digitally Tunable External Laser Cavity of Alternative Embodiments 1-4Coupled with Advanced Wavelength Modulation Techniques

Absorption spectroscopy with diode lasers traditionally relies on thetechnique of wavelength (frequency) modulation. This technique proved inthe past to be one of the most sensitive spectroscopic detectiontechniques in existence [e.g., Bjorklund, G. C. “Frequency ModulationSpectroscopy: a new method for measuring weak absorptions andDispersions”, Opt. Lett. Vol. 5, 15 (1980)]. With this approach, thewavelength is modulated in the vicinity of the absorption line of thesample under investigation, making the demodulated detector outputproportional to the derivative of the absorption line shape. Thewavelength modulation technique is therefore also known as derivative orharmonic spectroscopy. The first (or any odd) derivative can be used forlaser wavelength stabilization, by locking the laser to the zero valueof the derivative waveform, which corresponds to the peak of theabsorption line profile. The second (or any even) harmonic of thedetected waveform produces a peak proportional to the peak of theabsorption line at its center wavelength, and is typically used foranalytical measurements such as absorber concentration in the sample.The modulation bandwidth of the disclosed external cavity laser, atseveral tens of kHz, is high enough for typical wavelength modulationspectroscopy application, since it is above the bulk of theenvironmental noise. This makes the tuning/modulation mechanism of thedisclosed external cavity laser well suited for wavelength modulationspectroscopy, with the advantage of wide and fast spectral coverage forwavelength modulation.

Advanced wavelength modulation with the disclosed laser can be performedby modulating different wavelengths of interest at differentfrequencies, so that signals at multiple wavelengths specific to theanalyte may be combined, resulting in a higher signal-to-noise ratio.These wavelengths might also include wavelengths detecting interferents,whose second derivative signals could be generated with the oppositephase, and thus subtracted from the analyte signal at the demodulationstep, leading to yet another form of compressed spectral sensing withthe disclosed laser source.

The ability of the disclosed digitally controlled external cavity laserto access any wavelength at random opens up the possibility to performderivative spectroscopy in which non-adjacent wavelengths are modulatedduring the generation of the derivative signal. This feature representsa potentially superior detection modality with laser spectrometers.

Application: Infrared Spectrometer for Analytical Spectrometry

Widely tunable IR laser sources with random wavelength access and withthe capability for advanced modulation techniques can be used as basicbuilding blocks for advanced analytical instruments used in analyticalchemistry, both in laboratory and field applications. Infrared quantumcascade lasers (QCL) built into the subject digitally tunable cavitywould produce analytical instruments that surpass currently standardanalytical spectrometry equipment based on Fourier Transform Infrared(FTIR) technology. Digitally tunable external cavity QCLs offer betterdetection sensitivity, higher speed, smaller size, and are mechanicallymore stable for field use. FIG. 9 illustrates spectrometer 150comprising a subject external cavity digitally tunable laser 152 undercomputer control 154. The output is provided to sample 156. Detector 158creates the spectrometer output.

Application: Infrared Spectral Imaging Microscope

For applications in reflectance or transmittance microscopy,particularly related to biology and biomedical applications, it isdesirable to have a laser source that scans over several tens of laserwavelengths in the mid- and long-wave IR range within a time interval ofseveral milliseconds. The digitally controlled external cavity laserdisclosed here provides such performance. The laser provides fast (<0.1ms per step) wavelength tuning, stable and reproducible wavelengthlocking and fast (>30 kHz) wavelength modulation that can be used foradvanced signal detection algorithms. High optical power available froma digitally tunable quantum cascade laser (QCL) will provide thespectral imaging microscope with a high signal-to-noise ratio, even forsamples with weak absorption bands. The design and construction of alaser-based microscope has been traditionally based on two-dimensionalscanning of the laser beam over the sample. However, due to the highpower available from QCLs, flood illumination of the entire samplesimultaneously becomes possible, resulting in dramatically increasedspeed of data acquisition. FIG. 10 illustrates spectral imagingmicroscope 160 with a subject external cavity digitally tunable laser162 under computer control 164. The output is handled by optical system166 and provided to sample 168. Detector 170 creates the microscopeoutput.

Application: Spectrometer for Standoff Detection

A standoff spectroscopic detection instrument based on the subjectdigitally controlled external cavity laser can be built based onquantum-cascade gain elements. Recently, multiple-100 mW output quantumcascade lasers (QCL) were demonstrated in the mid- and long-wave IRrange (e.g., [Lyakh et al., 2010]). The standoff sensor uses scatteringat the hard surface of the target to generate the return signal.Depending on the output power used, the receiver detector can be cooledeither thermoelectrically or by cryogenic cooler.

The signal modulation/demodulation technique for the standoff sensor canbe based on the wavelength modulation approach, with spectral coveragein a wide spectral range and with advanced processing algorithms inorder to remove the effects of atmospheric and target reflectanceinterferents. Non-limiting examples of advanced in-hardware processingrelevant here are (1) derivative spectroscopy with non-adjacent “randomaccess” wavelengths and (2) matched filter. Previous work on a standoffsensor of methane as an indicator of leaks in natural gas pipelines[Vujkovic-Cvijin et al., 1999, 2001] demonstrated the extension of thetechnique of wavelength modulation spectroscopy from in situ absorptioncells to an open atmospheric path with natural target scatteringproviding the return beam.

Application: Trace Gas Spectrometer for In-Situ Monitoring

For trace gas detection in dynamic gas mixtures in the presence ofinterferents, several tens of laser wavelengths in the mid- andlong-wave IR range need to be scanned within several milliseconds. Thedigitally controlled external cavity laser with DMA modulation providesfast (<0.1 ms switching time) wavelength tuning, stable and reproduciblewavelength locking and fast (>30 kHz) wavelength modulation forderivative signal detection, as in traditional wavelength modulationspectroscopy. With the recently developed QCLs, it is possible to covera wide portion of the long-wave IR spectral range with the output powerof several tens to hundreds milliwatts per laser line, and with thespectral resolution of less than 1 cm⁻¹ to more than 10 cm⁻¹. In oneexemplary, non-limiting embodiment of the sensor, the tunablewavelength-modulated laser illuminates a photoacoustic cell, where theenergy absorbed in the target gas generates an acoustic signal detectedwith high sensitivity by phase-sensitive or equivalent detection. Due tothe unique capabilities of the subject digitally controlled externalcavity laser, sophisticated compressed sensing processing algorithmssuch as derivative detection with non-adjacent wavelengths and matchedfiltering can be implemented for data collection.

While the foregoing written description enables one of ordinary skill tomake and use what is considered presently to be the best modes thereof,those of ordinary skill will understand and appreciate the existence ofvariations, combinations, and equivalents of the specific embodiments,methods, and examples herein. The invention should therefore not belimited by the above described embodiments, methods, and examples, butby all embodiments and methods within the scope and spirit of theinvention.

What is claimed is:
 1. A sensor system, comprising: (a) a frequencyagile tunable laser that has an external cavity and an output, the lasercomprising: (i) one or more tunable gain elements providing sources ofoptical radiation at different wavelengths; (ii) a spectral dispersiondevice acting on the radiation produced by the gain elements to produceradiation dispersed into separate wavelengths; and (iii) a spatial lightmodulator (SLM) defining a plurality of programmable sub-apertures thatdetermine the wavelengths of the laser output, where the laser output isprovided to a sample; (b) a control system that applies a mathematicaltransform to the SLM to generate a detection filter for an analyte inthe sample, the detection filter comprising a set of laser outputwavelengths with defined intensities; and a detector that collects laseroutput after its exposure to the sample.
 2. The sensor system of claim 1wherein the control system controls the SLM so as to perform randomaccess in a time domain to any laser wavelength within the spectra ofthe gain elements.
 3. The sensor system of claim 1 wherein the controlsystem controls the SLM so as to perform chemical analysis or materialidentification in a short time period by reducing the number of laserwavelengths used, confining them to spectral regions of interest forcharacterizing one or more of the following: a specific analyte,expected interferents or a spectral background.
 4. The sensor system ofclaim 3 where the spectral regions of interest are not contiguous. 5.The sensor system of claim 1 wherein the control system controls the SLMso as to accomplish time-multiplexed modulation of multiple laserwavelengths, or a matched filter, or derivative spectroscopy, orderivative spectroscopy with non-adjacent wavelengths.
 6. The sensorsystem of claim 1 wherein the control system causes the SLMsub-apertures to change their properties so as to modulate at least twoor more different laser output wavelengths at different modulationfrequencies.
 7. The sensor system of claim 1 wherein the control systemis enabled to automatically perform spectroscopic signal processing byusing the SLM to generate sets of laser output wavelengths with definedintensities in order to create a detection filter with positive andnegative values, and wherein the control system implements the detectionfilter as two spectral templates, one representing a portion of thefilter with positive values, and one representing a portion of thefilter with negative values in order to achieve spectral matchedfiltering which distinguishes a specific analyte in contrast to theexpected interferents and background spectral features.
 8. The sensorsystem of claim 1 wherein the SLM comprises a digital micromirror array.9. The sensor system of claim 1 wherein the control system controls theSLM so as to generates a set of output laser wavelengths in order toenhance the detection sensitivity of the sensor system, or to reduce thenumber of laser wavelengths required or to reduce the time needed toperform chemical analysis or material identification, or to decrease thetotal laser power needed to perform chemical analysis or materialidentification.
 10. The sensor system of claim 1, wherein the controlsystem is enabled to automatically performs spectroscopic signalprocessing by causing the sup-apertures of the SLM to change theirproperties to select a set of non-contiguous wavelengths for use inchemical analysis or material identification; where the selectedwavelengths correspond to the spectral regions of interest of theanalyte and are selected to enhance the detection sensitivity of thesensor system, or to reduce the number of laser wavelengths required, orto reduce the time needed to perform chemical analysis or materialidentification, or to decrease the total laser power required.
 11. Thesensor system of claim 10 wherein the control system is enabled toautomatically perform spectroscopic signal processing by using the SLMto perform time-multiplexed modulation of multiple laser wavelengths.12. The sensor system of claim 10 wherein the control system is enabledto automatically perform spectroscopic signal processing by using theSLM to generate sets of laser wavelengths in order to perform derivativespectroscopy.
 13. The sensor system of claim 10 wherein the controlsystem causes the SLM to modulate at least two or more different laseroutput wavelengths at different modulation frequencies, allowing theapplication of synchronous signal detection technique to the detectionof the spectroscopic signal at two or more of the output wavelengthssimultaneously.
 14. The sensor system of claim 10 wherein the controlsystem is enabled to automatically perform spectroscopic signalprocessing by using the SLM to generate sets of laser output wavelengthswith defined intensities in order to create a detection filter withpositive and negative values, and wherein the control system implementsthe detection filter as two spectral templates, one representing aportion of the filter with positive values, and one representing aportion of the filter with negative values in order to achieve spectralmatched filtering which distinguishes a specific analyte in contrast tothe expected interferents and background spectral features.
 15. Thesensor system of claim 10 wherein the SLM comprises is digitalmicromirror array.